Wireless local area networks (LANs) have been developed as an enhanced replacement for wired LANs. In a wireless LAN for data-communication a plurality of (mobile) network stations (e.g., personal computers, telecommunication devices, etc.) are present that are capable of wireless communication. As compared to wired LANs, data-communication in a wireless LAN can be more versatile, due to the flexibility of the arrangement of network stations in the area covered by the LAN, and due to the absence of cabling connections.
Wireless LANs are generally implemented according to the standard as defined by the ISO/IEC 8802-11 international standard (IEEE 802.11). IEEE 802.11 describes a standard for wireless LAN systems that will operate in the 2.4-2.5 GHz ISM (industrial, scientific and medical) band. This ISM band is available worldwide and allows unlicensed operation for spread spectrum systems. For both the US and Europe, the 2,400-2,483.5 MHz band has allocated, while for some other countries, such as Japan, another part of the 2.4-2.5 GHz ISM ban has been assigned. The IEEE 802.11 standard focuses on the MAC (medium access control) and PHY (physical layer) protocols for AP based networks and adhoc networks.
In AP based wireless networks, the stations within a group or cell can communicate only directly to the AP. This AP forwards messages to the destination station within the same cell or through the wired distribution system to another AP, from which such messages arrive finally at the destination station. In ad-hoc networks, the stations operate on a peer-to-peer level and there is no AP or (wired) distribution system.
The 802.11 standard supports three PHY protocols: DSSS (direct sequence spread spectrum), FHSS (frequency hopping spread spectrum), and infrared with PPM (pulse position modulation). All these three PHYS provide bit rates of 1 and 2 Mbit/s. Furthermore, IEEE 802.11 includes extensions 11a and 11b which allow for additional higher bit rates: Extension 11b provides bit rates 5.5 and 11 Mbit's as well as the basic DSSS bit rates of 1 and 2 Mbit/s within the same 2.4-2.5 GHz ISM band. Extension 11a provides a high bit rate OFDM (orthogonal Frequency Division Multiplexing modulation) PHY standard providing bit rates in the range of 6 to 54 Mbit/s in the 5 GHz band.
The IEEE 802.11 basic MAC protocol allows interoperability between compatible PHYs through the use of the CSMA/CA (carrier sense multiple access with collision avoidance) protocol and a random back-off time following a busy medium condition. The IEEE 802.11 CSMA/CA protocol is designed to reduce the collision probability between multiple stations accessing the medium at the same time. Therefore, a random back-off arrangement is used to resolve medium contention conflicts. In addition, the IEEE 802.11 MAC protocol defines special functional behaviour for fragmentation of packets, medium reservation via RTS/CTS (request-to-send/clear-to-send) polling interaction and point coordination (for time-bounded services).
Moreover, the IEEE 802.11 MAC protocol defines Beacon frames sent at regular intervals by the AP to allow stations to monitor the presence of the AP. The IEEE 802.11 MAC protocol also gives a set of management frames including Probe Request frames which are sent by a station and are followed by the Probe Response frames sent by an available AP. This protocol allows a station to actively scan for APs operating on other frequency channels and for the APs to show to the stations what parameter settings the APs are using.
Every DSSS AP operates on one channel. The number of channels depends on the regulatory domain in which the wireless LAN is used (e.g. 11 channels in the U.S. in the 2.4 GHz band). The number can be found in ISO/IEC 8802-011, ANSI/IEEE Std 802.11 Edition 1999-00-00. Overlapping cells using different channels can operate simultaneously without interference if the channel distance is at least 3. Non-overlapping cells can always use the same channels simultaneously without interference. Channel assignment can be dynamic or fixed. Dynamic channel assignment is preferable, as the environment itself is dynamic as well.